1. Technical Field
The present invention relates to a method for providing a satellite orbit ephemeris, a server, and a positioning system.
2. Related Art
A GPS (global positioning system) is widely known as a positioning system using positioning signals, and used in a positioning device included in a cellular phone, a car navigation system, and the like. The GPS measures positions using a positioning calculation that calculates four parameters of three-dimensional coordinates indicating the GPS and clock error based on information such as positions of plural GPS satellites and pseudo distances between the respective GPS satellites and the GPS.
According to the position measurement by the GPS, satellite information such as positions, speeds, moving directions, and the like of the GPS satellites is obtained based on navigation data such as almanac and ephemeris data superimposed on GPS satellite signals transmitted from the GPS satellites, and positioning calculation is performed based on the satellite information and time information. Since the ephemeris data are particularly valuable keys to capturing the satellites, the time to first fix (TTFF) increases when positioning is started with no obtained ephemeris data, for example. Even when it is not a first fix, the capturing time considerably varies depending on whether the ephemeris of the corresponding satellite is obtained or not when the satellite is captured for the first time.
U.S. Pat. No. 7,142,157 discloses a server client system technology that includes a server for predicting an ephemeris for a long term such as one week and providing the predicted ephemeris (hereinafter referred to as the “long-term predicted ephemeris” (satellite orbit ephemeris)) to a positioning device as a client.
According to the technology disclosed in U.S. Pat. No. 7,142,157, data on predicted satellite orbit of all GPS satellites for all terms are provided to the positioning device as the long-term predicted ephemeris. However, the positioning device does not necessarily use all of the data in practical measurement. This is because only a limited number of the GPS satellites disposed on each of six rotation orbit surfaces and rotating around the earth can be observed from the position of the positioning device at a certain time. In this case, the data not used by the positioning device become useless and waste memory.